1. Field of the Invention
The present invention relates to an engine control system improved in accuracy of detecting an air-fuel ratio in a vehicle equipped with an internal combustion engine capable of carrying out feedback control of the air-fuel ratio.
2. Description of the Related Art
Hitherto, concentration of oxygen in exhaust gas of the internal combustion engine (hereinafter referred to as engine) installed in the vehicle is measured with an exhaust gas sensor, and a ratio of air to fuel (hereinafter referred to as air-fuel ratio) of air-fuel mixture supplied to the engine is controlled on the basis of the measured oxygen concentration in order to purify the exhaust gas and improve fuel consumption. In order to mass-produce automobiles and make them stably work over a long term of years, it is necessary to use exhaust gas sensors of stable operation that do not widely vary in characteristic and show less aged deterioration due to use. Therefore, several technical attempts to compensate the variation in characteristic and aged change and modify the characteristic have been heretofore developed and laid open to the public.
For example, in a generally known technique, an electric heater is used in combination with the exhaust gas sensor in general, and a gas-sensing portion of the exhaust gas sensor is controlled to be at an appropriate activation temperature by monitoring internal resistance of the exhaust gas sensor or the electric heater when the exhaust gas sensor is used.
The exhaust gas sensors vary in detection characteristic of the air-fuel ratio to the oxygen concentration and change in characteristic with the passage of time depending upon each individual gas sensor. Further, internal resistance, on which the temperature control is based, also varies in characteristic and changes with the passage of time.
The Japanese Patent Publication (unexamined) No. 1999-281617 titled xe2x80x98Gas Sensor, Connector of Gas Sensor, and Gas Concentration Detecting Devicexe2x80x99 discloses a technique to compensate variation in detection characteristic of the sensors by attaching a calibration resistor to each exhaust gas sensor.
The Japanese Patent Publication (unexamined) No. 1998-169500 titled xe2x80x98Output Compensating Device of Air-Fuel Ratio Sensorxe2x80x99 discloses an output compensating device of an air-fuel ratio sensor that includes an air-fuel ratio sensor for detecting air-fuel ratio of gas in exhaust passage of the engine, output detection means for detecting output of the air-fuel ratio sensor when the gas in the exhaust passage of the engine is in a state of a predetermined air-fuel ratio, and output compensation means for compensating output of the air-fuel ratio sensor on the basis of the output detected by the mentioned output detection means. The state of the foregoing predetermined air-fuel ratio indicates a state of atmospheric environment during cutting the fuel or during stopping the engine.
The Japanese Patent Publication (examined) No. 1992-24657 titled xe2x80x98Method and Apparatus for Measuring Temperature of Limit Current Zonde or xcex Zondexe2x80x99 discloses that internal resistance of an exhaust gas sensor is detected in the form of temperature detecting means at the time of controlling an electric heater for keeping the exhaust gas sensor at an appropriate activation temperature.
The Japanese Patent Publication (unexamined) No. 1989-172746 titled xe2x80x98Apparatus for Controlling Heater Temperature of Oxygen-concentration Sensorxe2x80x99 discloses that internal resistance of an electric heater is detected in the form of temperature detecting means at the time of controlling an electric heater for keeping the exhaust gas sensor at an appropriate activation temperature.
The Japanese Patent Publication (unexamined) No. 2001-349864 titled xe2x80x98Temperature Detector for Exhaust Gas Sensorxe2x80x99 discloses variation-calibrating means in internal resistance of exhaust gas sensors depending upon each individual product.
In every foregoing prior arts, a problem exists in that characteristic of the exhaust gas sensor that has varied with the passage of time is not compensated and, moreover, it is necessary to keep the environmental temperature of the exhaust gas sensor at a predetermined value with accuracy in order to avoid any error in oxygen-concentration data.
Moreover, although it is possible to carry out initial calibration of the exhaust gas sensor without using expensive calibration gas, it is not possible to compensate the variation in characteristic depending upon products and change in characteristic due to the passage of time.
Even if the internal resistance detecting means aiming at temperature control is improved, it is not possible to compensate the variation in characteristic depending upon products and the change in characteristic due to the passage of time.
In detecting the internal resistance for temperature control, the calibration means for calibrating the variation in characteristic of products and change in characteristic due to the passage of time depends on outside air temperature of the vehicle. Therefore, a further problem exists in the prior arts that it is difficult to correctly calibrate the temperature characteristic of a high-temperature activation region. Even if the temperature is correctly controlled; it is yet not possible to accurately detect the oxygen concentration because the sensors vary in oxygen-concentration detection characteristic and the characteristics change with the passage of time.
The present invention has been made to solve the above-discussed problems, and a first object of the invention is to provide an engine control system provided with -an exhaust gas sensor including a calibration resistor for oxygen-concentration detection output (hereinafter referred to also as oxygen-concentration data) the engine control system being capable of obtaining accurate oxygen-concentration data regardless of any change in oxygen-concentration detection characteristic due to the passage of time and any variation in internal resistance for temperature control depending upon products and in internal resistance due to the passage of time.
A second object of the invention is to provide an engine control system capable of detecting deterioration in characteristic of the exhaust gas sensor and automatically displaying a warning.
An engine control system according to the invention includes:
an exhaust gas sensor having a sensor element that is provided with an electric heater for controlling temperature near the sensor element and appropriately operating at a predetermined activation temperature, measuring oxygen concentration of exhaust gas of the engine and outputting oxygen-concentration data, and outputting predetermined oxygen-concentration data when the mentioned exhaust gas is in an atmospheric air exchange state and the temperature near the mentioned sensor element is at the predetermined activation temperature;
a standard characteristic storage memory in which a functional expression or a data table showing a relation between the mentioned oxygen-concentration data and an air-fuel ratio of the mentioned engine at the mentioned predetermined activation temperature and oxygen-concentration data in the mentioned atmospheric air exchange state are stored;
atmospheric air judging means for judging an atmospheric air state to determine that the mentioned exhaust gas is in an atmospheric air exchange state that the exhaust gas has been exchanged with the mentioned atmospheric air when fuel supply to the mentioned engine has been stopped exceeding a predetermined time;
first heater control means that controls the mentioned electric heater so that the mentioned oxygen-concentration data coincide with the oxygen-concentration data stored in the mentioned standard characteristic storage memory when the mentioned atmospheric air state judging means judges that the exhaust gas is in the atmospheric air state;
calibration signal reading means that stores internal resistance of the mentioned exhaust gas sensor or internal resistance of the mentioned electric heater at the point of time when the oxygen-concentration data outputted by the mentioned exhaust gas sensor being controlled by the mentioned first heater control means comes to coincide to the mentioned standard value data as a target internal resistance;
second heater control means that operates when fuel is supplied to the mentioned engine to control the foregoing electric heater so that a current value of the measured internal resistance of the mentioned exhaust gas sensor or the electric heater coincides to the mentioned target internal resistance; and
air-fuel ratio calculating means that has a microprocessor and calculates the current air-fuel ratio on the basis of the current oxygen-concentration data of the mentioned exhaust gas sensor controlled by the mentioned second heater control means and the mentioned functional expression or data table stored in the mentioned standard characteristic storage memory.
In the engine control system according to the invention, while keeping the oxygen-concentration data in the atmospheric air state at a calibration initial value at all times, and the air-fuel ratio is calculated on the basis of the detected oxygen-concentration data, and fuel supply is controlled so that the calculated air-fuel ratio becomes the target air-fuel ratio. As a result, it is possible to avoid influence of variation in characteristic of the exhaust gas sensor and the electric heater and change in characteristic due to the passage of time by carrying out initial calibration on the oxygen-concentration data.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.